This study is a multi-center randomized, double-blind placebo-controlled multiple dose, dose-escalation cohort protocol to evaluate the safety and preliminary biological efficacy of a novel antibiotic, a synthetic protegrin analogue, IB-367, delivered to the lower respiratory tract of patients with cystic fibrosis by jet nebulization. Protegrins are a class of endogenous (naturally occurring) antibiotics found in several mammalian species and exhibit wide-ranging potent bacterial killing effects. They are salt-insensitive (unlike defensins) making them attractive candidates for novel antibiotic therapeutic agents in cystic fibrosis, where some evidence for high salt concentrations in airway surface fluids have been reported. Evidence of antimicrobial efficacy in this protocol has been sought by quantitative microbiology of cultures obtained from expectorated sputum (counting bacteria). Safety measures have included multiple serial pulmonary function tests.